As a result of experience with long term operation and multiple reloads of nuclear fuel elements, it has been found that certain operating conditions arise which tend to reduce energy output per unit of fuel obtainable and thereby affect operating costs and efficiencies in an undesirable manner.
One of the problems in connection with the operation of nuclear reactors is the accumulation of debris of various sizes, which may occur during original construction, subsequent operation or during repair and may include nuts, bolts, metal turnings, shavings, wires, drill bits, electrical connectors, and various kinds of trash. During the operation of nuclear reactors, the metal debris which may be present in the nuclear reactor can be carried by the cooling water and can impact upon fuel assembly components. The repeated interaction of such debris and fuel assembly components can result in fretting damage to the components. Certain sizes of this type of debris are particularly troublesome, since that debris is likely to be carried by cooling water to the area near the bottom (lower ends) of the fuel rods. Some of the debris can be caught between the fuel rods and other fuel assembly components. The debris vibrates in the moving coolant and impacts principally upon the lower end of the fuel rods, ultimately abrading and causing fretting wear of the fuel rod cladding at that point. This type of wear is recognized as a significant cause of fuel failures or the escape of fuel and fission products leading to the premature withdrawal from service of the fuel assembly.
Attempts have been made in the past to mitigate the affect of such debris on the fuel rods by incorporating integral filters or integral filter mechanisms into the design and manufacture of new fuel assemblies containing unirradiated nuclear fuel. Such attempts to mitigate the debris problem in unirradiated or new fuel assemblies may unacceptably increase the pressure drop through the fuel assembly and can result in a lower coolant flow and, therefore, lowered permissible power level. However, the problem of protecting irradiated nuclear fuel rods in a nuclear fuel assembly has been more problematic and costly due to pressure drop considerations as well as the difficulty of modifying the assembly, remotely, and in a radioactive environment.